A year ago, American intelligence believed that Syrian President Bashar al-Assad was close to the end of his career, if not his life. Last March, at a press conference with King Abdullah of Jordan in Amman, Barack Obama said: “I’m confident that Assad will go. It’s not a question of if, it’s when.”

What about “never”? Assad’s war against his people is set to begin its fourth year. American officials, speaking anonymously, lately have been suggesting that he’ll remain in office for the foreseeable future and must be “re-engaged.”

Obama made his rash prediction in Amman before learning that Iran had ordered Hezbollah, Tehran’s Lebanese-based satellite, to send thousands of trained fighters to help Assad’s regime. Since then, Assad has used poison gas against Syrian civilians and raised the estimated death count of the war to about 130,000.

Hussein Suleiman, a commander in Ahrar Al-Sham, one of the country’s biggest rebel groups, had gone to negotiate with leaders of the ultra-conservative Islamic State of Iraq and the Levant (ISIS) about a local dispute in Maskanah, near Aleppo.

His dead body was later returned to comrades with an ear missing and visible signs of torture.

The Islamic Front, an alliance of Islamist groups including Ahrar Al-Sham, condemned the killing and attacked ISIS strongholds in the north in retaliation.

“They kidnapped him and tortured him and then killed him and disfigured his corpse, in a way unknown to the Syrian people prior to the revolution, even when it came to the branches of the criminal Assad regime’s security bodies,” said a statement from the Islamic Front.

Hassan Aboud, the political leader of the group, told Al Jazeera, “ISIS denies reality, refusing to recognize that it is simply another group. It refuses to go to independent courts; it attacked many other groups, stole their weapons, occupied their headquarters and arbitrarily apprehended numerous activists, journalists and rebels. It has been torturing its prisoners. These transgressions accumulated and people got fed up with ISIS. Some of those people have attacked ISIS’s positions, but ISIS was first to attack in other places, bringing this on itself.”

Widespread anger at the repressive and arbitrary nature of ISIS’s methods has been growing since the group’s arrival in northern Syria in May 2013 to fight the regime of Bashar Al-Assad. Many of the Islamist groups want the formation of an Islamic state in Syria. But ISIS goes further, calling for the restoration of the caliphate, an Islamic kingdom, across the Levant, a vast swathe of the eastern Mediterranean.

ISIS members have gained a reputation for brutal punishments for failing to comply with their strict interpretation of Islamic law.

In the town of Kafranbel, Muhammad Khatib, an activist, said ISIS “tried to impose their ideology on the Kafranbel people. They kidnapped activists and destroyed the media centre. They banned smoking, demanded Islamic attire for all the girls. They tried to ban jeans and casual clothes and asked men to grow beards. The people were fed up in less than a month.”

Since the killing of Mr. Hussein last week the Islamic Front and two other rebel coalitions, the Syrian Revolutionary Front (SRF) and Jaysh al-Mujahideen, have attacked ISIS throughout the regions of Aleppo and Idlib. Fighting has spread to other towns where tensions have been bubbling for months, including the border town of Azaz.

Sunday saw some of the most sustained and violent clashes between the rival rebel groups. In the town of Manbij, rebels seized a compound garrisoned by ISIS, activists said. The Britain-based Syrian Observatory for Human Rights monitoring group said ISIS fighters used car bombs, a tactic usually reserved for attacking government forces, for the first time to defend its territory.

In the town of Tal Rafaat, north of Aleppo city, insurgents from ISIS ambushed a rebel convoy, killing at least 14 fighters.

Dozens of fighters on each side have been killed and many more injured. There were reports that 59 ISIS fighters were killed on Sunday, There have also been reports of prisoners being killed en masse by ISIS fighters as they abandon positions or defect to the rebels.

ISIS is not officially linked to Al-Qaeda, but is closely affiliated with the terrorist group and shares a common ideology. Speaking in Israel last week, U.S. secretary of state John Kerry described ISIS as “the most dangerous players in this region.”

Syrian activists have been quick to call this the “second revolution,” but the Islamic Front has left the door open for a negotiated peace with ISIS.

“We would like these [ISIS] brothers to join their brethren in the Syrian revolution,” Mr. Aboud of the Islamic Front said. “We see them as nothing but another group. They see themselves as a state. They need to drop this illusion that they have come to believe as an established fact. It causes them to treat allies as opponents.”

While the outburst of fighting against ISIS is significant, its current impact on the trajectory of the broader Syrian conflict is unclear. At the moment, it doesn’t appear to have wider repercussions, said Charles Lister, a visiting fellow at the Brookings Doha Center who closely follows the conflict.

“For now, this simply represents three days of inter-factional fighting with an overtly anti-ISIS foundation,” he said. “Should ISIS launch a determined counter-attack, then this could come to represent a definitive moment in the Syrian conflict.

“No matter what takes place in the coming days and weeks, ISIS will remain in Syria in some form and should it be entirely isolated by all other key fighting groups in Syria, it’s actions will likely become even more harsh than before.”